Just your typical interview problem template :)

IteratorIterator.java

import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

/**
 * An iterator that flattens an iterator of iterators.
 *
 * Iterator SQUARED!
 */
public class IteratorIterator<T> implements Iterator<T> {
    
    public <T> IteratorIterator(Iterable<? extends T>... iterators) {
        /* 
         * I guess we should really be doing something with all these
         * here iterators...
         */
    }

    @Override
    public boolean hasNext() {
        /*
         * Should return false if and only if we've returned everything
         * up to and including the last element of the last non-empty
         * iterator.
         */
        return false;
    }

    @Override
    public T next() {
        /*
         * Return the current element and iterate the cursor to the next 
         * element of the first remaining non-empty iterator.
         *
         * You can safely assume that next() will NEVER be called if
         * hasNext() would return false.
         */
        return null;
    }

    @Override
    public void remove() {
        // DON'T WORRY ABOUT THIS ONE!!!
    }


    /**
     * The below is meant to be a test case for your implementation above. If done correctly,
     * it should print out the digits from 1 to 10, one per line, without any blank lines.
     */
    public static void main(String... args) {
        List<Integer> a = Arrays.asList(new Integer[] { });
        List<Integer> b = Arrays.asList(new Integer[] {1, 2, 3});
        List<Integer> c = Arrays.asList(new Integer[] { });
        List<Integer> d = Arrays.asList(new Integer[] { });
        List<Integer> e = Arrays.asList(new Integer[] {4});
        List<Integer> f = Arrays.asList(new Integer[] { });
        List<Integer> g = Arrays.asList(new Integer[] {5, 6, 7, 8, 9, 10});
        List<Integer> h = Arrays.asList(new Integer[] { });

        IteratorIterator<Integer> it = new IteratorIterator<Integer>(a, b, c, d, e, f, g, h);
        while (it.hasNext()) {
            System.out.println(it.next());
        }
        System.out.println("Done!");
    }
}